1. Field of the Invention
The present invention relates to an optical fiber coupler having wavelength multi- and demultiplexing functions.
2. Description of the Art
The optical fiber coupler with wavelength multi- and demultiplexing functions is a device for multiplexing and demultiplexing light waves of different wavelengths traveling through optical fibers.
FIG. 2 is a diagram schematically showing a conventional wavelength multiplexing/demultiplexing optical fiber coupler. In the Figure, reference numerals 1a and 1b designate optical fibers; 2, an optical coupling portion; 3, a protecting member; and 4, an adhesive portion. The method, which is most popular now, to manufacture the multiplexing/demultiplexing optical fiber coupler is a melt-extension method. This method is described in NEW GLASS, Vol. 6, No. 1, 1991, pp. 48 to 59. In this method, the covers of a plurality of optical fibers are partially removed. The uncovered portions or glass portions of the optical fibers are joined and bonded together by heating the exposed glass fibers by burner, for example. The bonded glass portion is extended and further heated to form an optical coupling portion 2. The extending process is continued until a desired characteristic (branch ratio) of the coupler is obtained. The characteristic of the optical fiber is measured during the extending process in such a way that light is input to one end of one of the optical fibers 1a and 1b, and light output from the other ends of the optical fibers is detected. Finally, the extended, bonded-portion of the optical fibers is fixed to a protecting members by adhesion. The optical fiber coupler is thus manufactured and is shown in FIG. 2.
The optical fiber coupler manufactured as stated above has a wavelength demultiplexing function. When a light wave of the wavelength .lambda.1 and another light wave of the wavelength .lambda.2 are simultaneously input at an input port, light of .lambda.1 emanates from an output port 1 and light of .lambda.2 emanates from an output port 2. In the description to follow, intensity of the light incident on the input port is denoted as P.sub.0, an intensity output from the output 1 as P.sub.1, and an intensity output from the output port 2 as P.sub.2.
FIG. 3 is a graph showing the characteristic of the conventional wavelength multiplexing/demultiplexing optical fiber coupler, which was described in connection with FIG. 2. In FIG. 3, a broken line indicates P.sub.1 - P.sub.0, and a solid line, P.sub.2 - P.sub.0. At a wavelength .lambda.1, the light intensity P.sub.1 is large, while the light intensity P.sub.2 is small. This fact implies that most of the wavelength .lambda.1 is output from the output port 1, and that most of the light of the wavelength .lambda.2 is output from the output port 2.
A degree of the wavelength demultiplexing capability is called a cross talk. A large cross talk indicates a high wavelength demultiplexing capability. In FIG. 3, the difference between the broken line and the solid line at the wavelength .lambda.1 indicates a magnitude of the cross talk. In the fields of communication and sensors, the cross talk is desirably 15 dB or more.
The conventional optical fiber coupler of this type has a tendency that when the width .DELTA..lambda. (=.lambda.2-.lambda.1) of the wavelengths to be multiplexed is selected to be 100 nm or shorter, the cross talk is small.